Japanese Patent Application No. 2000-172298, filed on Jun. 8, 2000, is hereby incorporated by reference in its entirety.
The present invention relates to a semiconductor device that incorporates an electrostatic protection circuit, and a method of fabrication thereof.
The application of a surge voltage due to static electricity to a signal output terminal, signal input terminal, or signal input-output terminal of a semiconductor device could lead to destruction of internal circuitry. For that reason, it is usual to connect a protective circuit to such signal terminals.
In this case, the gate isolation films of the MOS transistors being protected are becoming thinner as micro-processing techniques become ever finer, and the gate withstand voltage (gate dielectrics breakdown voltage) is decreasing.
With processing at the 0.35 xcexcm or 0.25 xcexcm level, the gate withstand voltage is still comparatively high, so that breakdown of a Zener diode occurs when a static charge is applied, which acts as a trigger to cause snapback in an NPN bipolar transistor due to the bipolar operation thereof, making it possible to protect again static charges by imposing a voltage clamp state.
With processing below 0.18 xcexcm, however, the gate withstand voltage decreases even further, so that the response characteristic is improved even further and thus it is not possible to protect a MOS transistor in this way without discharging static rapidly.
In such a case, an electrostatic protection circuit has been proposed that uses a thyristor instead of an NPN bipolar transistor, to employ the self-amplification operation of the thyristor (such as in Japanese Patent Application Laid-Open No. 9-293881, by way of example).
However, a thyristor is formed of a connection between an NPN bipolar transistor and a PNP bipolar transistor, it has a large number of terminals and the surface area thereof is larger, and thus it cannot be applied to micro-processing. Taking the above-mentioned Patent Application as an example, five electrically isolated impurity diffusion regions are required, which increases the surface area.
In addition, a prior-art electrostatic protection circuit has a resistor in the wiring path that is necessary for applying a static charge to a Zener diode, but, since the Zener diode cannot be made to break down rapidly, there is room for further improvement from the viewpoint of the response characteristic.
A further problem lies in the fact that the amplification capability of a bipolar transistor is dependent on the base length thereof, so that a bipolar transistor having a base length that is long in the lateral direction of the substrate, as in the prior art, would have a low amplification capability, and there is room for further improvement in that the capability of a PNP bipolar transistor is even lower.
An objective of the present invention is to provide a semiconductor device having an electrostatic protection circuit that can be applied to micro-processing at 0.18 xcexcm or less and which has a small surface area.
Another objective of the present invention is to provide a semiconductor device having an electrostatic protection circuit that has a good response, by causing a Zener diode to break down immediately after a static charge is applied, to act as a trigger for a thyristor.
A further objective of the present invention is to provide a semiconductor device having an electrostatic protection circuit that has a good response, by improving the amplification capabilities of bipolar transistors that form a thyristor.
A semiconductor device in accordance with a first aspect of the present invention relates to a semiconductor device having an electrostatic protection circuit such that static charge of a positive polarity that is applied to a signal terminal is discharged to a VSS power line side.
This semiconductor device comprises:
a semiconductor substrate;
a p-type and an n-type well regions formed on the semiconductor substrate;
a first p-type impurity diffusion region formed in a surface layer of the p-type well region;
a first n-type impurity diffusion region which is formed in a surface layer of the p-type well region and which is electrically isolated from the first p-type impurity diffusion region;
a second p-type impurity diffusion region which is formed in a surface layer of the p-type well region and which is electrically isolated from the first n-type impurity diffusion region;
a second n-type impurity diffusion region which is formed in a surface layer of the p-type and n-type well regions adjacent to the second p-type impurity diffusion region;
a third n-type impurity diffusion region connected to under surfaces of the second p-type and second n-type impurity diffusion regions in the p-type well region;
a third p-type impurity diffusion region connected to an under surface of the third n-type impurity diffusion region; and
a low-resistance layer formed on a surface of the second p-type and n-type impurity diffusion regions.
A Zener diode is formed by a p-n junction between the third n-type and p-type impurity diffusion regions; an NPN bipolar transistor is formed by the first n-type impurity diffusion region, the p-well region, and the n-well region; and a PNP bipolar transistor is formed by the second p-type impurity diffusion region, the third n-type impurity diffusion region, and the third p-type impurity diffusion region.
In addition, the signal terminal is connected to the second p-type and n-type impurity diffusion regions through the low-resistance layer, and the VSS power line is connected to the first p-type and n-type impurity diffusion regions.
A semiconductor device in accordance with a second aspect of the present invention relates to a semiconductor device having an electrostatic protection circuit such that static charge of a negative polarity that is applied to a signal terminal is discharged to a VDD power line side.
The semiconductor device in accordance with the second aspect of the present invention can be fabricated by substituting n-type components for the p-type components of the semiconductor device in accordance with the first aspect of the invention, and substituting p-type components for the n-type components thereof.